Silicon is the principle material used in the making of diodes and transistors for the power electronics market. Unfortunately, while silicon is low cost, it is not very efficient. A larger energy bandgap enables a material to withstand a strong electric field, making it possible to use a thinner device for a given voltage. A thinner device is highly desirable because it can have a lower on-resistance and higher efficiency. Although the weakness of silicon power electronics is addressed by forming devices in wide bandgap silicon carbide (SiC) and gallium nitride (GaN), such devices have had limited commercial success to date because they are much more expensive to fabricate and have also exhibited significant reliability issues.
Ultrawide bandgap semiconductors (UWBGS) are rapidly emerging as important alternatives to silicon, however, because they enable electrical devices that operate at significantly higher voltages, frequencies and temperatures. UWBGS have become leading contenders for use in next-generation devices for general semiconductor use.
Gallium oxide (Ga2O3) is an example of an ultrawide bandgap oxide semiconductor having excellent chemical and thermal stability up to 1400° C. Gallium oxide (Ga2O3) has a band gap of 4.5-4.9 eV, which is much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). It also exhibits high transparency in both the deep ultraviolet (DUV) and visible wavelength regions due to its very large bandgap. Furthermore, it has a transmittance of over 80% in the ultraviolet (UV) region. The monoclinic β-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. As a result, β-Ga2O3 is a promising material candidate for use in high-power electronics, as well as solar-blind photodetectors. In addition, single-crystal β-Ga2O3 substrates can be synthesized using scalable and low-cost melting-based growth techniques, such as edge-defined film-fed growth (EFG), floating zone (FZ) and Czochralski methods.
While formation of single-crystal β-Ga2O3 substrates is relatively straight-forward, the ability to grow high-quality and controllably doped β-Ga2O3 thin films has not been realized. Device-quality epitaxial thin films must have a high degree of purity, crystallinity, and be able to be doped controllably. Furthermore, these qualities much be achievable in films that can be grown at reasonable growth rates.
Prior-art growth techniques for β-Ga2O3 thin films been primarily focused on homo-epitaxy on commercially available Ga2O3 substrates using either molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE). Unfortunately, while conventional approaches have been successful in forming high-quality β-Ga2O3 thin films, reported growth rates have been too slow (˜2-10 nm/min.) for use in a practical device-manufacturing application.
In an effort to increase growth rate, growth of β-Ga2O3 films at atmospheric pressure using halide vapor-phase epitaxy (HVPE) using gallium chloride (GaCl) and oxygen (O2) as precursors was explored. Unfortunately, while higher growth rates were demonstrated (>5 μm/hour), the toxicity of the hydrochloric acid and chlorine sources used in the HVPE growth process is undesirable. In addition, these sources can introduce deleterious defect levels and impurity centers in the resultant thin films.
The need for a method of forming high-quality gallium oxide films at high growth rates remains unmet in the prior art.